Download Fluorescence quenching induced by oxidizing conditions in the

FLUORESCENCE QUENCHING INDUCED BY OXIDIZING CONDITIONS IN THE GREEN
NON-SULFUR BACTERIUM CHLOROFLEXUS AURANTIACUS: A PROPOSED
ECOPHYSIOLOGICAL SIGNIFICANCE IN HOT SPRING MICROBIAL MATS
Alberto Vianelli, Deborah Armiento, Guido Domingo, Leonardo Leonforte, Paolo D.Gerola
Università degli Studi dell’Insubria, Dipartimento di Biologia Strutturale e Funzionale, via
J.H.Dunant 3, 21100 Varese, Italy
Excess excitation energy dissipation via non-photochemical quenching and its possible role in
photoprotection is currently under active investigation in photosynthetic eukaryotes, albeit
relatively little attention has been paid to its presence in photosynthetic prokaryotes. A major
goal of the work in our laboratory is to analyze redox-dependent fluorescence quenching in
chlorosomes-containing photosynthetic bacteria (green bacteria). In particular we focus on
Chloroflexus aurantiacus, a facultative phototroph isolated from hot spring microbial mats in
Japan and Yellowstone National Park. In our previos work, we have shown that under
oxidizing conditions, obtained by controlled addition of potassium ferri/ferrocyanide,.
bacteriochlorophyll (bchl) a and c fluorescence yield in isolated chlorosomes is controlled by
two types of redox-sensitive mechanisms. Aerobic redox titration indicated an apparent Em of
410 mV for a five-fold decrease, suggesting a novel mechanism, probably based on oxidizing
bacteriochlorophyll (Bchl+) itself acting as a quencher, is operating in this bacterium. This
mechanism is absent in green sulfur bacteria (Chlorobium sp.), contaning chlorosomes as well,
but otherwise adapted to a highly reducing environment (sulfide-rich hot springs). In the
present work, we have undertaken further studies in order to better characterize the
fluorescence quenching in chlorosmes, and to relate the in vitro observations in chlorosomes
to in vivo conditions: whole cells, isolated membranes, antenna complex LH1, reaction
centers were studied by means of steady state absorption and fluorescence emission spectra.
Finally, taking into account of redox equilibria in oxygenated waters, we propose a
physiological role for this oxidant-induced fluorescence quenching, in view of the extreme
environmental parameters (particularly oxygen concentration) variations observed in hot
spring microbial mats. It is possibly worthy of note that an increasing amount of data from
otehr labs point to a possible involvement of (B)chl + based quenching in highly oxidizing
conditions in cyanobacteria and higher plants too.